Modulating a signal for radio transmission involves the manipulation and allocation of two resources among a plurality of communicators; these resources are frequency and time. Division of a frequency spectrum, so that each pair of communicators is allocated part of the spectrum for all of time, is called “Frequency Division Multiple Access” (FDMA). Division of time, so that each of the communicators is allocated a large part, or all, of the spectrum for units of the divided time is called “Time Division Multiple Access” (TDMA). In Code Division Multiple Access (CDMA), every communicator will be allocated the entire spectrum all of the time, and each communicator is assigned a unique code.
CDMA employs unique spreading codes to spread base band data before transmission. The CDMA signal is transmitted in a channel, the transmission signal sent below noise level. The CDMA receiver then uses a correlator to de-spread the wanted signal, which is passed through a narrow band pass filter. Unwanted signals will not be de-spread and will not pass through the filter. Codes take the form of a carefully designed one/zero sequence produced at a much higher rate than that of the base band data. The rate of a spreading code is referred to as chip rate rather than bit rate.
TDMA operates by chopping up the transmission channel into sequential time slices. Each user of the channel takes turns transmitting and receiving in a round-robin fashion. In reality, only one communicator is actually using the channel at any given moment, however the communicator only uses the channel for short bursts of time. The communicator is then preempted to allow other communicators to transmit. The TDMA method is very similar to time slicing in a computer processor to run multiple applications simultaneously.
In contrast CDMA permits every communicator to transmit at the same time. Conventional wisdom teaches that this is simply not possible. Using conventional modulation techniques, it most certainly is impossible. What makes CDMA work is a special type of digital modulation called “spread spectrum”. This form of modulation takes the user's stream of bits and splatters them across a very wide channel in a pseudo-random fashion. The “pseudo” part is very important, since the receiver must be able to undo the randomization in order to collect the bits together in a coherent order.
As an analogy, consider a room full of people, all trying to carry on one-on-one conversations. In TDMA each pair of communicators takes turns talking. They keep their turns short by speaking only one sentence at a time. As there is never more than one person speaking in the room at any given moment, no one has to worry about being heard over the background noise. In CDMA, all pairs of communicators talk at the same time, but they all use a different language. Because none of the listeners understand any language other than that of the individual to whom they are listening, the background interference doesn't cause communications problems.